This invention relates to a modems. More particularly, this invention relates to modems which utilize handshake procedures and which require information (e.g. a password) which is not normally transmitted in a handshake, before data communication is permitted to be established. The invention is used with modems utilizing a CCITT V32 handshake procedure, although it it is not limited thereto.
Communications between modems are often governed by various standards, including standards or recommendations promulgated by the International Telegraph and Telephone Consultative Committee (CCITT). For example, a V.32 standard exists for full duplex modems which can transmit information at 9600 bits/second with a 4800 bits/second back-up. In the V.32 standard, start-up procedures (i.e. handshakes) are set forth in "Data Communication Over the Telephone Network--Recommendations of the V Series--Volume III--Fascile VIII.1", CCITT VIIIth Plenary Assembly, Malga-Torremolinos 9-19 October 1984, which is hereby incorporated by reference herein. The communication starts when an originating modem places a call and the answering modem detects a ring and goes off hook. As seen in FIG. 4/V.32 of the CCITT Recommendation which is reproduced as the prior art FIG. 1 hereof, the handshake continues when the answering modem sends an answer tone (ANS) of 2100 Hz for one or more seconds. The originating (or "calling") mode, upon receipt of the answer tone sends a 1800 Hz signal (AA), and the answering modem, upon receiving the AA signal responds with an AC signal (600 Hz and 3000 Hz tones), followed by a CA signal (continued tones reversed in phase) followed by another AC signal. Meanwhile, upon detecting the switch from AC to CA, the originating modem switches from signal AA to signal CC. In this manner each modem can measure the round trip delay experienced over the channel and can adjust its echo cancellers appropriately.
Once the round trip delay has been determined, the modems continue their handshake. The answering modem sends out a signal AB (S) for two hundred fifty-six symbol intervals, a signal CD (S) for sixteen symbol intervals, a training sequence (TRN) for between 1280 and 8192 symbol intervals, and a repeated sixteen bit rate sequence R1. The training sequence includes a pseudo-random sequence with scrambled ones at 4800 bits/second with dibits encoded directly to states A, B, C, and D according to a predetermined format. The function of the S, S, and TRN sequence being sent by the answering modem is to train the adaptive equalizer in the calling modem as well as to train the echo canceller to the answering modem. The sixteen bit rate sequence R1, on the other hand is a 4800 bits/second scrambled and differentially encoded sequence as set forth in Table 1/V.32 which is used to indicate to the calling model which of several modes the answering modem can support.
Upon receiving the S, S, TRN, and R1 sequence, the originating modem essentially repeats the same sequence, except that instead of supplying the R1 signal after the training sequence, the calling modem provides an R2 signal. The R2 signal indicates the common modes between the originating and answering modem, and provisionally chooses a mode. While the R2 signal continues, the answering modem again repeats the S, S, TRN, sequence, followed by R3 which finally chooses the mode of communication (e.g. 9600, 9600 Trellis encoded, 4800, 2400). The handshake terminates with respective transmissions by the originating and answering modem of an E signal which confirms the mode of communication, and a B1 signal which is comprised of scrambled marks at the agreed upon rate. The handshake terminates and desired data transmission starts immediately after the B1 signal.
As will be appreciated by a review of FIG. 1 and by reference to the CCITT recommendation which has been incorporated by reference herein, the V.32 handshake does not provide for passwords or the like which would provide built in security against unauthorized dial up access or against network tampering. In fact, with the provided V.32 handshake, the answering modem will communicate with any calling modem which has the proper V.32 handshake. Those skilled in the art will also appreciate that the handshakes set out by other standards similarly do not include passwords or other security against unauthorized dial up access. The handshakes of the art also do not typically envision the passing of any additional information during the handshake sequence.